1. Field of the Invention
Embodiments of the invention relate to a display device, and more particularly to a backlight unit of liquid crystal display device. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for preventing a hot spot phenomenon in which a position of a light source is so bright as to be recognized through a liquid crystal display panel.
2. Description of the Related Art
Generally, a liquid crystal display device controls light transmittance of liquid crystal cells in accordance with video signals to display a picture. An active matrix type of liquid crystal display device having a switching device provided for each liquid crystal cell is advantageous in the implementation of displaying moving pictures because each liquid crystal cell can be actively controlled. The switching device used for the active matrix liquid crystal display device employs a thin film transistor (hereinafter, referred to as “TFT”).
FIG. 1 is an equivalent circuit diagram of a pixel provided in the related art liquid crystal display device. Referring to FIG. 1, the active matrix LCD converts a digital input data into an analog data voltage on the basis of a gamma reference voltage and then supplies the analog data voltage to a data line DL while at the same time a scanning pulse is supplied to a gate line GL to thereby charge a liquid crystal cell Clc. A gate electrode of the TFT is connected to the gate line GL while a source electrode thereof is connected to the data line DL. Further, a drain electrode of the TFT is connected to a pixel electrode of the liquid crystal cell Clc and to one electrode of a storage capacitor Cst. A common electrode of the liquid crystal cell Clc is supplied with a common voltage Vcom.
The storage capacitor Cst is charged by a data voltage fed from the data line DL when the TFT is turned-on, thereby constantly keeping a voltage on the liquid crystal cell Clc. If the scanning pulse is applied to the gate line GL, then the TFT is turned on to provide a channel between the source electrode and the drain electrode thereof, thereby allowing a voltage on the data line DL to be supplied to the pixel electrode of the liquid crystal cell Clc. In this case, liquid crystal molecules of the liquid crystal cell have an alignment change due to an electric field between the pixel electrode and the common electrode, to thereby modulate an incident light passing through the liquid crystal molecules.
FIG. 2 is a block diagram showing a configuration of the related art liquid crystal display device. More particularly, FIG. 2 is block diagram of a configuration for a small-size liquid crystal display device used in a cell phone. Referring to FIG. 2, the related art liquid crystal display device 100 includes a liquid crystal display panel 110 provided with a thin film transistor TFT driving the liquid crystal cell Clc and positioned where data lines DL1 to DLm and gate lines GL1 to GLn cross each other, a data driver 120 supplying a data to the data lines DL1 to DLm of the liquid crystal display panel 110, a gate driver 130 supplying a scanning pulse to the gate lines GL1 to GLn of the liquid crystal display panel 110, a gamma reference voltage generator 140 generating a gamma reference voltage to supply it to the data driver 120, a backlight unit 150 for irradiating light onto the liquid crystal display panel 110, a common voltage generator 160 generating a common voltage Vcom to supply to the common electrode of the liquid crystal cell Clc of the liquid crystal display panel 110, a gate driving voltage generator 170 generating a gate high voltage VGH and a gate low voltage VGL to supply to the gate driver 130, and a timing controller 180 controlling the data driver 120 and the gate driver 130.
The liquid crystal display panel 110 has a liquid crystal molecules positioned between two glass substrates. On the lower glass substrate of the liquid crystal display panel 110, the data lines DL1 to DLm and the gate lines GL1 to GLn perpendicularly cross each other. TFTs are provided adjacent to crossings between the data lines DL1 to DLm and the gate lines GL1 to GLn. The TFTs supply data on the data lines DL1 to DLm to the liquid crystal cells Clc in response to scanning pulses. The gate electrodes of the TFTs are connected to the gate lines GL1 to GLn while the source electrodes thereof are connected to the data lines DL1 to DLm. Further, the drain electrode of the TFTs is connected to the pixel electrodes of the liquid crystal cells Clc and to the storage capacitors Cst.
The gamma reference voltage generator 140 receives a high-level supply voltage VDD to generate a positive gamma reference voltage and a negative gamma reference voltage and outputs them to the data driver 120. The common voltage generator 160 receives a high-level supply voltage VDD to generate a common voltage Vcom, and supplies it to the common electrode of the liquid crystal cell Clc provided in each pixel of the liquid crystal display panel 110.
The gate driving voltage generator 170 is supplied with a high-level supply voltage VDD to generate the gate high voltage VGH and the gate low voltage VGL, and supplies them to the gate driver 130. Herein, the gate driving voltage generator 170 generates a gate high voltage VGH higher than a threshold voltage of the TFT provided in each pixel of the liquid crystal display panel 110 and a gate low voltage VGL lower than the threshold voltage of the TFT. The gate high voltage VGH and the gate low voltage VGL generated in this manner are used for determining a high level voltage and a low level voltage of the scanning pulse generated by the gate driver 130, respectively.
The timing controller 180 supplies a digital video data RGB from a digital video card (not shown) to the data driver 120 and, at the same time, generates a data driving control signal DCC and a gate driving control signal GDC using horizontal/vertical synchronizing signals H and V in response to a clock signal CLK to supply them to the data driver 120 and the gate driver 130, respectively. Herein, the data driving control signal DDC includes a source shift clock SSC, a source start pulse SSP, a polarity control signal POL and a source output enable signal SOE, etc. The gate driving control signal GDC includes a gate start pulse GSP and a gate output enable signal GOE, etc.
The data driver 120 supplies a data to the data lines DL1 to DLm in response to a data driving control signal DDC supplied from the timing controller 180. Further, the data driver 120 samples and latches a digital video data RGB fed from the timing controller 180, and then converts it into an analog data voltage capable of expressing a gray scale level at the liquid crystal cell Clc of the liquid crystal display panel 110 on the basis of a gamma reference voltage from the gamma reference voltage generator 140. The analog data voltage is then supplied to the data lines DL1 to DLm.
The gate driver 130 sequentially generates scanning pulses, that is, gate pulses in response to a gate driving control signal GDC and a gate shift clock GSC supplied from the timing controller 180 and then supplies the scanning pulses to the gate lines GL1 to GLn. The gate driver 130 determines a high level voltage and a low level voltage of the scanning pulse in accordance with the gate high voltage VGH and the gate low voltage VGL supplied from the gate driving voltage generator 170. The TFTs are turned-on in response to scanning pulses applied, via the gate lines GL1 to GLn, to the gate terminals thereof. Upon turning-on of the TFTs, a video data on the data lines DL1 to DLm is supplied to the pixel electrodes of the liquid crystal cells Clc.
The backlight unit 150 is provided at the rear side of the liquid crystal display panel 110. The backlight unit 150 radiates light onto each pixel of the liquid crystal display panel 110 in response to an alternating current voltage and a current supplied from the inverter 160. The backlight unit 150 can be either edge type or direct type.
The related art backlight unit includes a plurality of light-emitting diodes that each irradiate light as a point light source. Since the light-emitting diodes and the liquid crystal display panel 110 are integral and integrated to an upper housing (not shown), such as in a cell phone, then the light-emitting diodes and the liquid crystal display panel 110 are positioned closely adjacent to each other unlike in large-sized electronics, such as a TV display panel that have a relatively large space between the light-emitting diodes and the liquid crystal display panel. Accordingly, a hot spot phenomenon can occur in small-sized electronics in which the positions of the light-emitting diodes are distinctively recognizable through the liquid crystal display panel 110 when the light-emitting diodes are radiating light. Such hot spots deteriorate the quality of images displayed on the liquid crystal display panel 110.